FIX Use DataColumns content when available in CSV export

[sig-steve]

Fix #9248 broke #9173 by accidentally checking for a string key in a
numerically indexed array. Change the check to search the array instead
(fixes #10308)

[GuySartorelli]

Thanks Steve! Can you please retarget this to the 4.10 branch? That way we can include this in a patch release and it can be used without waiting for the next minor.

[GuySartorelli]

The travis build erroring is due to the way the travis-shared-config is configured. There's a hardcoded workaround to make sure the current minor branch gets the correct dependencies - this is hardcoded because we're intending to move to github actions and the effort of making it dynamic with travis config was deemed not worth it, but it does cause problems such as this one when we're in beta.

I'll run the tests locally.

[sig-steve]

Done. Looks like Travis CI cannot pass as the test suite appears to be requiring silverstripe/framework ^4.11, which won't work for this patch on 4.10.

[GuySartorelli]

I've created a PR to resolve the travis mismatch in #10315 - once that is merged I'll ask you to rebase so we can see the tests go green.

[GuySartorelli]

@sig-steve The fix for the travis builds has been merged into 4.10 - can you please pull and rebase? That should allow the tests to pass.

[GuySartorelli]

@sig-steve Tagged as 4.10.6